The long-term aim of these studies is to determine the fundamental, molecular mechanisms by which length governs the time course and strength of contraction in the heart. Others have shown that myoplasmic calcium dynamics are unaffected by length, and so the microscopic mechanics of the sarcomere will be examined in order to identify those factors intrinsic to the myofibril in which an increase in length slows relaxation in an isolated heart muscle preparation. One specific goal is to determine if length's effect is a (1) purely mechanical consequence of a length-coupled in uniformity of sarcomere motions, (2) a delayed onset of submicroscopic sarcomeres yielding, (3) a true showing of the early phase of isometric tension decline. Sarcomere length will be observed by light diffraction and held constant by servo-feedback to eliminate the effects of net internal shortening during contraction. Simultaneously, (a) the behavior of the speckled pattern of light will be kymographically recorded and (b) the muscle will be observed with high speed cinemicroscopy in order to evaluate length's influence on submicroscopic motions which might be undetected by diffraction. Second, the shape of the sarcomere force-velocity relations will be inspected to discern whether alteration in active site numbers and/or dynamics (1) can account for length's influence on relaxation and (2) form the basis which triggers sarcomere yielding and depresses subsequent isometric force development in cardiac muscle.